When voltage is applied to an organic electroluminescence device (occasionally simply referred to as an organic EL device hereinafter), holes and electrons are injected into an emitting layer respectively from an anode and a cathode. The injected holes and electrons are recombined in the emitting layer to provide excitons. According to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. An organic EL device may be classified by the emission principle into a fluorescent EL device and a phosphorescent EL device. The internal quantum efficiency of a fluorescent EL device, which uses emission caused by singlet excitons, is supposed to be 25% at a maximum. In contrast, it is known than the internal quantum efficiency of a phosphorescent EL device, which uses emission caused by triplet excitons, can be improved up to 100% as long as intersystem crossing from singlet excitons is efficiently achieved.
A technology for extending a lifetime of a fluorescent organic EL device has recently been developed and applied to full-color displays of a mobile phone, TV and the like. However, a fluorescent EL device is required to be improved in efficiency.
In view of the above, a highly efficient fluorescent organic EL device using delayed fluorescence has been suggested and developed. For instance, an organic EL device using a triplet-triplet fusion (TTF) mechanism, which is one of mechanisms of delayed fluorescence, is suggested. The TTF mechanism uses such a phenomenon that two triplet excitons collide with each other to generate triplet excitons.
Theoretically, the delayed fluorescence based on the TTF mechanism is supposed to improve the internal quantum efficiency of fluorescence emission up to 40%. However, fluorescence emission is still required to be improved in efficiency as compared with phosphorescence emission. Accordingly, in order to improve the internal quantum efficiency, a device using another delayed fluorescence mechanism has been studied.
For instance, a thermally activated delayed fluorescence (TADF) mechanism has been studied. The TADF mechanism uses such a phenomenon that inverse intersystem crossing from triplet excitons to singlet excitons occurs when a material having a small energy difference between singlet energy level and triplet energy level is used. As for thermally activated delayed fluorescence, refer to, for instance, “ADACHI, Chihaya, ed. (Mar. 22, 2012), Yuki Hando-tai no Debaisu Bussei (Device Physics of Organic Semiconductors), Kodansha, pp. 261-262.”
For instance, Patent Literature 1 discloses an organic EL device using the TADF mechanism.
Specifically, Patent Literature 1 discloses an organic electroluminescence device in which an emitting layer contains an organic luminescent material emitting fluorescence and delayed fluorescence. According to Patent Literature 1, the organic luminescent material emitting delayed fluorescence may be a compound in which an indolocarbazole ring is bonded to a nitrogen-containing heterocycle. Especially, it is described that a compound having an energy difference (ΔE) between singlet energy and triplet energy of 0.2 eV or less (ΔE corresponds to ΔST described above) is preferable. When such a compound having a small ΔST is used, inverse intersystem crossing from a triplet energy level to a singlet energy level is caused by a heat energy. Theoretically, the delayed fluorescence based on the TADF mechanism is supposed to improve the internal quantum efficiency of fluorescent emission up to 100%.
Non-patent Literature 1 discloses that a compound containing an electron donating unit in the form of phenoxazine and an electron accepting unit in the form of 2,4,6-triphenyl-1,3,5-triazine enables efficient luminescence based on the TADF mechanism.